The present invention relates to a composition from which the solid cores and solid centers (collectively referred to below as “center cores”) of solid golf balls and wound golf balls, and also one-piece golf balls, can be efficiently produced and which are capable of conferring a high rebound, and to a golf ball obtained using such a composition and endowed with a high rebound. More particularly, the invention relates to a golf ball-forming rubber composition which is suitable for pressure molding and capable of producing molded parts having an adequate hardness and an excellent initial velocity performance, and to a golf ball produced using such a rubber composition and endowed with a high rebound.
Golf ball-forming rubber compositions have hitherto been used as materials for molding the center cores of solid golf balls and wound golf balls, and for molding one-piece golf balls.
Various improvements have been made to date in golf ball-forming rubber compositions. For example, JP-A 11-262544 discloses a method for producing rubber spheres wherein an organosulfur compound such as a mercapto group-containing benzothiazole or thiadiazole or an aromatic thiophenol (e.g., pentadichlorothiophenol) is blended into a composition containing at least a base rubber and a co-crosslinking agent so as to adjust the rubber composition to a material suitable for injection molding at a slow vulcanization rate.
However, the organosulfur compound in the foregoing disclosure is blended for the purpose of slowing vulcanization so as to give the rubber composition a vulcanization rate suitable for injection molding. Employing such a rubber composition in the pressure molding process customarily used to mold golf ball-forming rubber compositions will therefore increase the vulcanization time, lowering productivity.
To address this problem, JP-A 2001-276278 and corresponding U.S. Pat. No. 6,525,141 disclose the addition of a given amount of specific mercaptans to a rubber composition, making it possible to shorten the vulcanization time and thus improve productivity. Yet, even though the molded products thus obtained have a suitable hardness, the initial velocity remains low and there appears to be room for further improvement.
In addition, U.S. Patent Application No. 2004-102580 (which corresponds to JP-A 2004-168977) discloses art in which an organosulfur compound is added for the purpose of adjusting the trans structure content of the polybutadiene making up the core. However, here too, the initial velocity remains inadequate, leading to a desire for even further improvement.